Temperament refers to the stable moods and behavior profiles observed in infancy and early childhood that are influenced by biology rather than experience alone. Two temperamental categories of children called behaviorally inhibited and uninhibited are characterized by distinctive responses to unfamiliar people, objects, and situations in the second year of life. Children with an inhibited temperament are timid with people, objects and situations that are novel or unfamiliar, whereas uninhibited children spontaneously approach novel persons, objects and situations. We recently demonstrated with functional magnetic resonance imaging (fMRI) that adults who had been categorized in the second year of life as inhibited, compared with those who had been categorized as uninhibited, showed greater amygdalar activation to unfamiliar vs. familiar neutral faces. These fMRI results suggest that some brain differences due to temperament are preserved from infancy into adulthood. Moreover, an inhibited temperament is a risk factor for the later development of social anxiety disorder (SAD) or social phobia. Thus neuroimaging studies of SAD that do not control for temperamental biases persisting from infancy may confound brain activations related to an inhibited temperament with those associated with SAD. Furthermore, independent longitudinal studies in two different laboratories have revealed infant profiles at four months of age called high reactive and low reactive that biased to become, respectively, inhibited and uninhibited. We now propose to replicate and extend our previous psychopathological and fMRI findings, in an independent and larger cohort of 16-17 year olds who have been followed longitudinally, were carefully characterized with respect to temperament by direct laboratory based assessment as 4 month-old infants (high reactive/low reactive), and categorized again as toddlers (inhibited/uninhibited). This research has the overarching aim of evaluating the predictive relation between early temperamental biases, on the one hand, and on the other the development of social anxiety disorder and profiles of brain activation to theoretically relevant stimuli. The significance of these efforts for the mental health of adolescents is discussed.